GCOV

When the flag --gcov is set, the build is reconfigured to produce (and keep) *.gcno files.

For example:

$ tools/bazel build --gcov //common:kernel_aarch64

You may find the *.gcno files under the bazel-bin/<package_name>/<target_name>/<target_name>_gcno directory, where <target_name> is the name of the kernel_build() macro. In the above example, the .gcno files can be found at

bazel-bin/common/kernel_aarch64/kernel_aarch64_gcno/

... or in dist_dir:

$ tools/bazel run --gcov //common:kernel_aarch64_dist
[...]
$ ls bazel-bin/common/kernel_aarch64/kernel_aarch64_gcno/
[...]

Handling path mapping

After you boot up the kernel and mount debugfs:

$ mount -t debugfs debugfs /sys/kernel/debug

You may see gcno files under:

/sys/kernel/debug/gcov/<some_host_absolute_path_to_repository>/<some_out_directory>/common/<some_source_file>.gcno

To map between these paths to the host, consult the gcno_mapping.<name>.json under bazel-bin/.

GKI

In the above example, the file can be found after a build:

$ tools/bazel build --gcov //common:kernel_aarch64
[...]
$ cat bazel-bin/common/kernel_aarch64/gcno_mapping.kernel_aarch64.json
[...]

You may also find this file under dist_dir:

$ tools/bazel run //common:kernel_aarch64_dist -- --dist_dir=out/kernel_aarch64/dist
[...]
$ cat out/kernel_aarch64/dist/gcno_mapping.kernel_aarch64.json
[...]

Device mixed builds

You need to consult the JSON file for the device kernel_build. Using virtual device as an example, you may find the files under:

$ tools/bazel build --gcov //common-modules/virtual-device:virtual_device_x86_64
[...]
$ cat bazel-bin/common-modules/virtual-device/virtual_device_x86_64/gcno_mapping.virtual_device_x86_64.json
[...]

Or under dist_dir:

$ tools/bazel run --gcov //common-modules/virtual-device:virtual_device_x86_64_dist -- --dist_dir=out/vd/dist
[...]
$ cat out/vd/dist/gcno_mapping.virtual_device_x86_64.json
[...]

Note: You will also see gcno_mapping.kernel_x86_64.json under dist_dir. That file is incomplete as it does not contain mappings for in-tree modules specific for virtual device.

Sample content of gcno_mapping.<name>.json:

Without --config=local (see sandboxing):

[
  {
    "from": "/<repository_root>/out/bazel/output_user_root/.../__main__/out.../android-mainline/common",
    "to": "bazel-out/.../kernel_x86_64/gcno"
  }
]

With --config=local (see sandboxing):

[
  {
    "from": "/mnt/sdc/android/kernel/out/cache/.../common",
    "to": "bazel-out/k8-fastbuild/bin/common/kernel_aarch64/gcno"
  }
]

The JSON file contains a list of mappings. Each mapping indicates that the .gcno files located in <from> were copied to <to>. Hence, /sys/kernel/debug/<from> on the device maps to <to> on host.

Note: For both <from> and <to>, absolute paths should be interpreted as-is, and relative paths should be interpreted as relative to the repository on host. For example:

[
  {
    "from": "/absolute/from",
    "to": "/absolute/to"
  },
  {
    "from": "relative/from",
    "to": "relative/to"
  }
]

This means:

• Device /sys/kernel/debug/absolute/from maps to host /absolute/to
• Device /sys/kernel/debug/<repositry_root>/relative/from maps to host /<repository_root>/relative/to.